This project focuses on field studies designed to investigate the interactions between hydrology, mineralogy, geology and geochemistry that result in naturally-elevated As concentrations (>10 ug/L) in reducing groundwater. We will first seek to determine if a roughly linear relationship between groundwater As and groundwater age observed in Araihazar, Bangladesh, holds in New England. This new relationship, corresponding to a steady rate of As mobilization of approximately 20 ug/L per year, implies that the rate of recharge of an aquifer plays an important -perhaps dominant- role in regulating the spatial distribution of As in groundwater. Four areas in Maine and New Hampshire with known clusters of domestic wells with As concentrations up to 700 ug/L were selected for detailed study on the basis of bed-rock and surficial geology. Two new areas of Bangladesh with very high and low groundwater As concentrations, respectively, were selected for detailed investigation, including groundwater dating. Field observations from one site in New England and one site in Bangladesh will be synthesized with a reactive-transport model of a high-As plume along its flow path to a discharge area. There are striking similarities to the tectonic setting that led to the formation of the rocks that, eventually, produced the deposits of New England and Bangladesh where elevated groundwater As concentrations are observed. We propose to explore the implication of this analogy by conducting experiments designed to elucidate the mechanisms leading to the formation of mobilizable As in sediment. These experiments will include long-term laboratory and in situ incubations of model minerals (arsenopyrite and silicates), synthetic silicate glasses doped with arsenic, as well as rock specimens collected in the upland regions and river deposits from New England and Bangladesh. Mineralogical changes will be monitored by selective extractions, XRD, SEM-EDX, XAS-XANES and -EXAFS;microbiological changes will be tracked as well. We also propose to conduct push/pull experiments in the un-consolidated glacial-till aquifers tapped by the majority of public supply wells in New England to assess the mobility of As in response to changes in land use that could affect the subsurface hydrology or geochemistry.